1. Field of the Invention
The invention pertains to antenna pattern synthesis and more particularly to apparatus and method for obtaining a focal region feed distribution for realizing selected free space antenna patterns.
2. Description of the Prior Art
The far field pattern and the aperture field distribution of an antenna are Fourier transform pairs when the field in the aperture plane outside the aperture bounds is zero. This permits the pattern produced by an aperture to be readily analyzed. Pattern synthesis, however, presents a much more difficult problem. In practice, the far field pattern is generally specified only in amplitude and as a consequence thereof, the aperture distribution is not uniquely determined; the desired amplitude pattern being realizable from various combinations of amplitude and phase distributions in the aperture. For most high gain antennas, however, the aperture phase is either constant or linearly distributed across the aperture, resulting in radiated beams with linear phase fronts, the beams being directed along the perpendiculars to the linear phase fronts. Thus, for high gain antennas, each radiated pattern may be decomposed into a weighted sum of linear phase front beams, each of which has a linear phase distribution across the aperture, the superposition of amplitude and phase distributions determining the over-all aperture distribution to achieve the desired far field antenna pattern.
Aperture distributions that are uniform in amplitude and phase provide maximum aperture efficiency in that the maximum gain achievable with a given aperture is realized. When the phase distribution is altered from uniform to linear, the effective aperture is decreased becoming equal to the projection in the direction of the radiated beam, which direction is along the perpendicular to the linear phase front. Though uniform amplitude and phase distribution provides maximum aperture efficiency, the ratio of main beam amplitude to the maximum sidelobe amplitude for many applications is inadequate. To improve this ratio, various aperture distributions have been considered each exhibiting its limitations and attributes. The radiation patterns resulting from these distributions are sensitive to aperture phase and amplitude errors, exhibiting beam distortions and sidelobe level deteriorations with deviations from the prescribed aperture values. The minimization of these errors in reflector and lens type antennas requires manufacturing the reflector surfaces and lens elements to tight tolerances which impose a fundamental limitation on the sidelobe levels and significantly increase the cost of the antennas. The sidelobe level limitations may be overcome by constituting the aperture as a planar array of discrete elements and individually adjusting the phase and amplitude at each element by tedious experimental methods.
The present invention discloses an apparatus and method with which desired amplitude and phase distributions may be achieved in antenna apertures in a relatively simple manner with prevailing tolerance conditions.